1. Technical Field
The present invention relates to an amplification circuit amplifying a high-frequency signal.
2. Related Art
In known amplification circuit amplifying a high-frequency signal, a circuit that controls current feedback by varying resistance of a source of an FET is a semiconductor amplification element. For example, such an amplification circuit is disclosed in Japanese Unexamined Patent Application Publication No. 9-270643. As shown in FIG. 3, the amplification circuit includes a pair of amplifying FETs 42 and 43 to which input signals are input, a current feedback circuit 45 provided between sources of the pair of amplifying FETs 42 and 43, and voltage feedback circuits 46 and 47 provided between drains and gates of the amplifying FETs 42 and 43. Constant current circuits 48 and 49 are connected between the ground and the sources of the amplifying FETs 42 and 43, and current flowing in the amplifying FETs 42 and 43 is constantly controlled by the constant current circuits 48 and 49.
The current feedback circuit 45 is configured by subsequently connecting, in three stages, a plurality of serial circuits in which two fixed resistors and a switching FET are serially connected. Gates of switching FETs 51, 52, and 53 of the serial circuits are connected to a control circuit 55. Any one of the switching FETs 51, 52, and 53 is turned on by a control of the control circuit 55, and thus a plurality of fixed resistors R21 to R26 is selectively connected in series, thereby varying a resistance value of current feedback resistors of the pair of amplifying FETs 42 and 43.
The voltage feedback circuits 46 and 47 include fixed resistors R27 and R28 provided between the drains and the gates of the amplifying FETs 42 and 43, respectively, and negatively feed a part of output signals back to the input side through the fixed resistors R27 and R28. In the amplification circuit, the resistance of the current feedback circuit 45 is controlled to vary gain reduction, a part of the output signals in the voltage feedback circuits 46 and 47 are negatively fed back to the input side, and thus a gain characteristic is flat with respect to frequency of the output signals.
However, the amplification circuit 41 has a problem that input impedance increases and thus return loss of input signals deteriorates, when gain reduction is to be increased as shown in FIG. 4. Thus, it is difficult to widen the range of amplification rates by increasing gain reduction.